Hey,
I have one customer who wants to set up email forwarding like:

email1@domain.com -> email1@his_isp.com

email2@domain.com -> email2@his_isp.com

email_whatever@domain.com -> email_whatever@his_isp.com

so every email send to the specific mailbox at domain.com will be forwarded to the same mailbox on another server.

the problem is how to do this without adding every email forwarder manually ?

I use Cpanel/WHM server.

If you don't do it "manually", how do you expect it to get done?

If you want to speed the process up, just edit the domains valiase file. vi /etc/valiases/domain.com

Format is like this:

user@domain_on_server.com: user@other_domain.com
user2@domain_on_server.com: user2@other_domain.com

At the bottom once you have added all of your specific forwarders setup your "catchall" entry:

*: user@other_domain.com